The invention relates to a flashlight emission apparatus, and more particularly, to such apparatus which automatically controls the power supply to a main discharge capacitor.
As is well recognized, a conventional flashlight emission apparatus which is used for flash photography includes a main discharge capacitor charged to a high voltage, the discharge of which triggers a flash discharge tube to emit light. The emission of flashlight from the tube is enabled when the capacitor is charged to a level which exceeds a discharge initiate voltage of the tube.
Also, a conventional flashlight emission apparatus includes a neon tube which becomes illuminated to indicate the completion of a charging operation whenever the main capacitor has been charged to a level which is sufficient to enable the discharge tube to produce a given amount of emission.
FIG. 1 is a circuit diagram of an electrical circuit which may be used in a conventional flashlight emission apparatus. The apparatus essentially comprises a battery E1 of a low voltage; a DC-DC converter 1 including a feedback transistor Tr1, an oscillating transistor Tr2, a step-up transformer T1 and rectifier diode D1; a main discharge capacitor C2; a neon tube Ne1 which indicates the completion of a charging operation; a trigger circuit 2 including synchro-contacts SW2, a trigger capacitor C3 and a trigger transformer T2; and a flash discharge tube Xe1.
The positive terminal of the battery E1 is connected to one end of a primary coil of the transformer T1 while its negative terminal is connected to the emitter of the oscillating transistor Tr2 of an NPN type through a main switch SW1. The collector of this transistor is connected to the other end of the primary coil of the transformer T1, and its base is connected through a resistor R2 to the collector of the feedback transistor Tr1 of a PNP type. The transistor Tr1 has its emitter connected to the positive terminal of the battery E1 and its base connected to one end of a secondary coil of the transformer T1 and also connected to the junction between the resistor R1 and capacitor C1. The other end of the resistor R1 is connected to the negative terminal of the battery E1 through the switch SW1 while the other end of the capacitor C1 is connected to the positive terminal of the battery E1. The other end of the secondary coil of the transformer T1 is connected to the anode of the diode D1, the cathode of which is connected through a resistor R3 to one end of a neon tube Ne1 and also connected to one end of a main discharge capacitor C2. The other end of the tube Ne1 is connected to a common bus or ground line l.sub.0 which is connected to the negative terminal of the battery E1 through the main switch SW1, as is the other end of the main capacitor C2.
The flash discharge tube Xe1 and the trigger circuit 2 which triggers the tube Xe1 are connected in shunt with the main capacitor C2. Specifically, a series combination of a resistor R4 and synchro-contacts SW2 is connected in shunt with the capacitor C2, with the junction therebetween being connected through the trigger capacitor C3 to one end of a primary coil of the trigger transformer T2, the other end of which is connected to the common bus l.sub.0 and also connected to one end of a secondary coil of the transformer T2. The other end of the secondary coil is connected to a trigger electrode of the discharge tube Xe1.
In the described arrangement, when the main switch SW1 is closed, the transistors Tr1 and Tr2 in the DC-DC converter 1 are repeatedly turned on and off to provide an oscillating or intermittent current flow through the primary coil of the transformer T1, with result that a high voltage is induced across the secondary coil thereof and is applied to the capacitor C2 through the diode D1. The voltage across the main capacitor C2 increases gradually in a manner graphically illustrated in FIG. 2, and when such voltage reaches a discharge initiate voltage V.sub.MIN of the discharge tube Xe1, the emission of flashlight from the tube Xe1 is enabled even though the neon tube Ne1 has not yet been illuminated. As the charging operation continues and the voltage across the main capacitor C2 reaches a voltage level V.sub.PRO representing a proper level for emission of flashlight from the discharge tube Xe1, the neon tube Ne1 initiates its discharge, thus indicating the completion of the charging operation of the main capacitor C2. If the charging operation is further continued, the voltage across the main capacitor C2 asymptotically approaches a maximum voltage V.sub.MAX which is determined by the turns ratio between the primary and the secondary coil of the transformer T1.
However, in a conventional flashlight emission apparatus as described above, the emission of flashlight from the discharge tube Xe1 is enabled if the voltage across the main capacitor C2 exceeds the initiate voltage V.sub.MIN at t.sub.1 even before it reaches the proper voltage V.sub.PRO at t.sub.2. Accordingly, if the discharge tube Xe1 should be triggered at time between t.sub.1 and t.sub.2, it will discharge to emit flashlight of an amount which is insufficient to provide a proper exposure. This leads to a difficulty that a user may be unaware of the resulting underexposure, thus missing the chance to take another picture with a proper amount of exposure.
Also in a conventional flashlight emission apparatus as described above, as long as the main switch SW1 remains closed, the converter 1 continues its booster operation even after the voltage across the main capacitor C2 has reached the proper voltage level V.sub.PRO to cause the neon tube Ne1 to continue its discharge, thus resulting in a wasteful dissipation of the capacity of the battery E1. If a user inadvertently forgets to turn the main switch SW1 off, the battery E1 may be exhausted as a result of the discharge of the neon tube Ne1, making the flashlight emission apparatus useless when required.